Graphite bodies have potential for use in a variety of applications, including uses in nuclear reactors, electrochemical fuel cells, the production of silicon and polysilicon, and other applications where a non-reactive material like graphite is needed. One characteristic missing from conventional graphites, however, is the combination of high porosity and ultra-low permeability which permits impregnation of materials into the graphite but which prevents leakage across the graphite article. It is this unusual combination of characteristics which permits the graphite of the present disclosure to be used in applications such as nuclear reactors, electrochemical fuel cells, the production of silicon and polysilicon, etc.
Graphite articles may be fabricated by combining calcined petroleum coke and coal-tar pitch binder into a stock blend. In this multi-step process, the calcined petroleum coke is first crushed, sized and milled into a finely defined powder. Generally, particles up to about 25 millimeters (mm) in average diameter are employed in the blend. The particulate fraction preferably includes coke powder filler having a small particle size. Other additives that may be incorporated into the small particle size filler include iron oxides to inhibit puffing (caused by release of sulfur from its bond with carbon inside the coke particles), coke powder and oils or other lubricants to facilitate extrusion of the blend.
The stock blend is heated to the softening temperature of the pitch and is form pressed to create a “green” stock body such as a plate. For green body production, a continuously operating extruding press may be used to form a plate known as a “green” body. For graphite article production, the green body is formed by die extrusion or by molding in a forming mold to form a “green body.”
The green stock body is heated in a furnace to carbonize the pitch so as to give the body permanency of form and higher mechanical strength. Depending upon the size of the graphite body and upon the specific manufacturer's process, this “baking” step requires the green body to be heat treated at a temperature of between about 700° C. and about 1100° C. To avoid oxidation, the green stock body is baked in the relative absence of air. The temperature of the body is raised at a constant rate to the final baking temperature. In some embodiments, the green stock body is maintained at the final baking temperature for between 1 week and 2 weeks, depending upon the size of the body.
After cooling and cleaning, the baked body may be impregnated one or more times with coal tar or petroleum pitch, or other types of pitches known in the industry, to deposit additional pitch coke in any open pores of the body. Each impregnation is then followed by an additional baking step, including cooling and cleaning. The time and temperature for each re-baking step may vary, depending upon the particular manufacturer's process. Additives may be incorporated into the pitch to improve specific properties of the graphite body. Each such densification step (i.e. each additional impregnation and re-baking cycle) generally increases the density of the stock material and provides for a higher mechanical strength. Typically, forming each body includes at least one densification step. Many such articles require several separate densification steps before the desired density is achieved.
After densification, the body, referred to at this stage as a carbonized body, is then graphitized. Graphitization involves heat treatment at a final temperature of at least about 2600° C., and generally (but not necessarily) up to about 3400° C., for a time sufficient to cause the carbon atoms in the calcined coke and pitch coke binder to transform from a poorly ordered state into the crystalline structure of graphite. At these high temperatures, elements other than carbon are volatilized and escape as vapors.
After graphitization is completed, the body can be cut to size and then machined or otherwise formed into its final configuration. Given its nature, graphite permits machining to a high degree of tolerance, thus permitting a strong connection between graphite plates or the like.
The lengthy densification cycles greatly increase the expense and time of manufacture of graphite bodies. For example, it may take about six months to form certain graphite articles, depending on the number of densification steps. Other graphite articles may take about 35 days to manufacture, again depending on the number of densification steps. More to the point, the graphite articles produced by this conventional processing do not have the high porosity, low permeability characteristics sought for certain applications.